code: def _get_aux_wire(aux_wire, tape, device_wires):
      r"""Determine an unused wire to be used as auxiliary wire for Hadamard tests.
  
      Args:
          aux_wire (object): Input auxiliary wire. May be one of a variety of input formats:
              If ``None``, try to infer a reasonable choice based on the number of wires used
              in the ``tape``, and based on ``device_wires``, if they are not ``None``.
              If an ``int``, a ``str`` or a ``Sequence``, convert the input to a ``Wires``
              object and take the first entry of the result. This leads to consistent behaviour
              between ``_get_aux_wire`` and the ``Wires`` class.
              If a ``Wires`` instance already, the conversion to such an instance is performed
              trivially as well (also see the source code of ``~.Wires``).
          tape (pennylane.tape.QuantumTape): Tape to infer the wire for
          device_wires (.wires.Wires): Wires of the device that is going to be used for the
              metric tensor. Facilitates finding a default for ``aux_wire`` if ``aux_wire``
              is ``None`` .
  
      Returns:
          object: The auxiliary wire to be used. Equals ``aux_wire`` if it was not ``None``\ ,
          and an often reasonable choice else.
      """
      if aux_wire is not None:
          aux_wire = qml.wires.Wires(aux_wire)[0]
          if aux_wire in tape.wires:
              msg = "The requested auxiliary wire is already in use by the circuit."
              raise qml.wires.WireError(msg)
          if device_wires is None or aux_wire in device_wires:
              return aux_wire
          raise qml.wires.WireError(
              "The requested auxiliary wire does not exist on the used device."
          )
  
      if device_wires is not None:
          if len(device_wires) == len(tape.wires):
              raise qml.wires.WireError(
                  "The device has no free wire for the auxiliary wire."
              )
          unused_wires = qml.wires.Wires(
              device_wires.toset().difference(tape.wires.toset())
          )
          return unused_wires[0]
  
      _wires = tape.wires
      for _aux in range(tape.num_wires):
          if _aux not in _wires:
              return _aux
  
      return tape.num_wires
  
file_path: pennylane/pennylane/gradients/metric_tensor.py
imports: from typing import Sequence, Callable
  import functools
  from functools import partial
  import warnings
  import numpy
  import pennylane
  from pennylane.circuit_graph import LayerData
  from pennylane.queuing import WrappedObj
  from pennylane.transforms import transform
label_desc: Use image node_id 9 for calling a global function with example usage: _get_aux_wire(aux_wire, tape, device_wires) and returns: tape, unused_wires, aux_wire, _aux